I. Field of the Invention
The present invention relates generally to industrial hot air solder leveling of printed circuit boards. In particular, the present invention relates to a vertically oriented system that solders circuit boards while they are captivated by spaced apart holders. Related prior art may be found in U.S. Class 118, subclasses 56, 63, 424 and 425 among others.
II. Description of the Prior Art
As will be recognized by those skilled in the art, printed circuit boards are typically mass soldered in a conventional process involving a solder bath during manufacture. Solder coating machines coat the board with a layer of solder as the board is quickly dipped in or otherwise passed through a hot, liquid solder bath. Optionally, such coating systems may employ pneumatic knives that clean boards as they emerge from the bath by removing excess solder. The knives typically blow molten solder away from desired through holes and the like.
The known prior art includes systems in which the boards are either horizontally or vertically inserted into the solder bath. Conventional horizontal systems move the board through the solder bath between controlled pairs of drive rollers that guide the board in and out of the bath. Examples of horizontal circuit board soldering systems are shown in U.S. Pat. Nos. 4,227,518, 5,007,369, 5,038,706 and 5,209,782. All of the devices described in the aforementioned patents solder the printed circuit board as it travels horizontally across spaced apart rollers in the solder bath.
However, horizontal soldering machines often do not perform satisfactorily. Problems experienced with horizontal soldering machines include board breaking because of undue stress during soldering, inefficient solder adhesion on the top board surface and the like. Thus, vertically oriented solder machines have recently become popular with some printed circuit board manufacturers.
Vertical systems typically secure the circuit boards during processing with a clamp that grips the top of the board. For example, U.S. Pat. No. 4,706,602, describes a clamp for holding circuit boards at one edge. Alternatively, some vertical systems employ drive rollers as well as shown in U.S. Pat. No. 5,226,964. Regardless of the clamping apparatus, the system ordinarily dips the board into a hot, liquid solder bath and then removes it after a predetermined residence time.
While vertical systems overcome several of the problems associated with horizontal systems, particularly complete board coating, they are not without unique problems of their own. For one, known vertical soldering machines often experience a stability problem with large circuit boards. In other words, the larger the board being soldered, the less stable mechanically it becomes as it traverses the solder bath. This instability causes the board to vibrate or waffle as it leaves the bath, resulting in imprecise solder adhesion and/or non-uniform solder adhesion. Since the circuits themselves often require precision to the millionth of an inch in solder thickness, board instability is often a significant problem.
The inherent structural instability of the larger boards often directly contributes to the lower precision in their soldered connections because of the stresses involved in the soldering process. For example, boards held on only one edge often vibrate as they are raised from the solder bath. This vibration problem is exacerbated by the conventional air knives employed to remove excess or entrained solder from the boards. As the boards vibrate, the connections on their surfaces are subjected to varying air flows and the thickness of the connections correspondingly vary as well. This varying thickness causes imprecise component mounting in the assembly process. As may well be imagined, such imprecision adversely affects board performance.
One obvious method of alleviating the stability problem is to simply increase the clamp grip area. In other words, the larger the grip area of the clamp, the greater the stability. However, as the clamp grip area increases, less circuit board surface is available for soldering (i.e., circuitry space is reduced). Further, modern circuit boards are lightweight and thin. The thinner the board, the more likely it is to experience damage from the clamping mechanism, as clamping forces are concentrated upon only a narrow area of the board. Since the board is grasped at a top edge by a large clamp in a conventional vertical system, it can subject a portion of the board to extreme stress. Further, since most clamps are pneumatically operated and the boards are generally plastic resin, the boards are comparatively fragile and easily broken by the clamps.
One means of addressing the stability problem is to add some form of dynamic holding apparatus to the clamp system that grips the bottom of the circuit board. For example, if both diametrically opposed corners of a board could be grasped by the clamping mechanism, greater stability would result. However, in the prior art when the latter approach has been employed, problems can occur. For example, systems employing such clamping mechanisms immerse the bottom clamp member in the solder bath. When the board is withdrawn from the bath, excess hot solder may be splashed about the machine. Splashing can damage the machine and injure an operator. Additionally, splashing wastes valuable solder, and slows down the manufacturing process.
Therefore it is desirable to provide soldering means whereby a printed circuit board may be securely clamped at opposed ends to stabilize the board during the soldering process. An ideal soldering machine would use an added clamping apparatus that prevented wasteful and potentially dangerous solder splashing during board removal. An ideal solution would stabilize the boards while preventing splashing and minimizing board damage to maximize system throughput.